Our research focus of epithelial and stromal cell specific gene expression signatures of normal and malignant human prostate tissues has led to the development of more precise CaP gene discovery and validation platforms, molecular bio-specimen resources and important new discoveries. Most notable of these promising findings highlighted ETS related gene (ERG) overexpression as a common feature (60-70%) of the epithelial cell transcriptome of prostate cancers. Subsequently ERG was shown to be a common fusion partner in prevalent TMPRSS2 and ETS related gene fusions leading to the overexpression of ERG through the androgen receptor (AR) regulated TMPRSS2 gene promoter in CaP. Accumulating evidence from various experimental models and human prostate cancers from our and other laboratories now generally support causal nature of ERG oncogenic activation in CaP. The central hypothesis of this proposal is that unscheduled androgenic activation of the ERG in prostate is causal in CaP development and/or progression. The goals of the proposal will be achieved by following aims: Aim 1. Development of new insights into mechanisms of ERG activation in CaP. ERG functions will be defined in cell culture and mouse prostate reconstitution models. Effects of ERG knock-down or ectopic expression of the full length TMPRSS2-ERG products commonly detected in CaP will be evaluated for biological features of cell differentiation and mechanism of regulating ERG transcriptional targets: PSA, NKX3.1, MSMB and SLC45A3. Our original information on ERG splice variants in CaP will be translated into a new transgenic mouse model to assess the assess effects of androgenic activation of mouse Erg on prostate differentiation and cancer. Aim 2. Delineation of ERG and C-MYC cooperativity in prostate tumorigenesis. We shall define biologic and biochemical mechanisms of regulation of C-MYC by ERG and cooperative effects of C-MYC and ERG in abrogating prostate epithelial differentiation programs. Mice overexpressing ERG and C-MYC have been generated to evaluate biological effects of these two critical genes in prostate epithelial differentiation and malignancy. Aim 3. New strategies to define clinical utility of the ERG alterations in stratification and prognosis of CaP. We will evaluate the unprecedented specificity of ERG MAb (>99.9%) for detecting ERG positive tumor cells in enhancing CaP diagnosis and assessing biologic behavior of ERG positive CaP. We will evaluate prognostic value of detecting ERG alterations in combination with C-MYC, AR PTEN, and AKT levels in prostate tumors. Significance: New insights into the CaP associated ERG functions and highly specific ERG detection in clinical specimens will provide will break new grounds in developing biologic stratification of CaP and new therapeutic strategies.